(i) Field of the Invention
This invention relates to an improved process for the annealing of drawn carbon steel rolls and coils of carbon steel sheet.
(ii) Description of Related Art
One of the sectors of the steel processing industry is that of drawing.
Drawing steel comprises passing the steel through a die, which has a specific cross-section which may be circular, square, hexagonal, etc., in order to obtain a piece of great length and constant cross-section which is identical to the cross-section of the die. This piece, which is obtained by cold forming, is obtained by applying a specific continuous pulling force to the end.
A lubricant, which is normally sodium stearate or calcium stearate, is applied to the material in order to ease the passage of the steel being shaped through the die, immediately before it passes through the die.
This lubricant adheres firmly to the entire surface of the part and its entire length.
As the pieces obtained by drawing are very long, on leaving the drawing machine they are made into rolls of a specific weight, which assists subsequent handling.
Pieces obtained by cold drawing undergo changes in their crystalline structure and mechanical properties as a result of this process and cannot always be used in that state in industry.
Most frequently these pieces, as rolls, are subjected to annealing heat treatment, recrystallization heating, to restore the structure of drawn pieces and adjust their mechanical properties for subsequent processes in the industry.
When these pieces have been hot drawnxe2x80x94wire drawingxe2x80x94they are also subjected to globulizing heat treatment.
The annealing treatment consists of progressively heating the drawn steel rolls from ambient temperature to a specific temperature above 700xc2x0 C., and once the mass of steel in the rolls reaches the specified temperature they are allowed to cool slowly to ambient temperature.
If the necessary precautions are not taken during the process of annealing the drawn steel rolls, the steel can lose some of the carbon which it contains, and if this occurs and the loss of carbon is greater than that accepted in the specifications, the quality of the steel suffers and it cannot be used for the task for which it was intended.
This loss of carbon can only be caused by:
reaction between the carbon in the steel and oxygen,
reaction between the carbon in the steel and carbon dioxide,
reaction between the carbon in the steel and water, in the vapour state.
Of the three reactions mentioned, the one having the most marked effects and the one which is the most difficult to avoid is the last one shown, that is the reaction between the carbon in the steel and water vapour.
Necessary precautions are taken in the industry to avoid these risks of carbon loss, the solution most widely used being the use of inert atmospheres, in which the component is nitrogen, or slightly reactive atmospheres in which the basic component is nitrogen with very small concentrations of a hydrocarbon, normally natural gas or propane.
In this way, and with normal annealing cyclesxe2x80x94continuous heating up to the specified annealing temperaturexe2x80x94reactions between the carbon in the steel and oxygen and carbon dioxide are successfully prevented, but not the reaction between the carbon in the steel and water vapour, so there is a loss of carbon from the steel after treatment, and this despite a relatively high consumption of atmosphere in relation to the quantity of steel annealed.
It is relatively easy to eliminate the species which oxidize the carbon in the steel, oxygen (O2) and carbon dioxide (CO2), and this can be done without difficulty before the outer coils of the steel rolls reach the temperature of 680xc2x0 C., which is the temperature at which the reaction between the carbon in the steel and the previously mentioned species which oxidize it, O2, CO2 and H2O, begins.
This is not the case however for water vapour (H2O), which is also another oxidizing agent for the carbon in the steel, and which furthermore has more marked oxidizing effects.
In the normal cycles for annealing drawn steel rolls, when heating is continuous up to the annealing temperaturexe2x80x94always above 680xc2x0 C.xe2x80x94the following take place:
When the outer turns of the drawn steel rolls reach a temperature of 680xc2x0 C. the centre or core of the rolls is still at a lower temperature. This temperature gradient can be 25 to 50xc2x0 C. depending upon the size of the rolls.
This temperature gradientxe2x80x94with colder cores in the rollsxe2x80x94is sufficient to ensure that not all the moisture in the centres or cores of the steel rolls has been eliminated.
This moisture leaves the core of the steel rolls, and coming into contact with the outer turns of the rolls at a temperature of 680xc2x0 C. or more reacts with the carbon and gives rise to a loss of carbon or decarburization of the material.
The purpose of this invention is to improve the process described above, for which purpose it is proposed that the following four operations be performed:
modification of the annealing cycle during the heating stage,
continuous analysis of the quantity of H2O present in the atmosphere (within the furnace and in direct contact with the drawn steel rolls),
holding for a sufficient time for the removal of water to reach a level such that the partial pressure of the said water is sufficiently low for there to be no risk that this water will react with the carbon in the steel, and
strongly reducing the flow of atmosphere when the analysed H2O level presents no risk of reaction with the carbon in the steel.
By proceeding in this way drawn steel rolls can be annealed without any loss of carbon and with a smaller total consumption of atmosphere than in the normal annealing processes known hitherto.
Thus one object of this invention is a process for annealing drawn carbon steel rolls in which the steel being shaped, to which a lubricant has previously been applied, is caused to pass through a die of specified cross-section, producing a roll by cold forming which is subsequently subjected to annealing heat treatment by progressively heating the drawn roll up to a specified temperature, after which the entire roll is allowed to cool slowly to ambient temperature, characterized in that in the heating stage heating of the steel roll is stopped before the temperature at which the reaction between the carbon in the steel and H2O in the vapour phase begins, avoiding the said reaction between the carbon in the steel and the water vapour, after which the temperature of the core of the steel roll is allowed to become the same as that in the outer part of the roll, with the progressive removal of moisture from the core of the steel roll, until a minimum moisture level is reached which guarantees a sufficiently low partial pressure to avoid the reaction between the carbon in the steel and the water vapour, after which heating is continued to the desired annealing temperature, thus obtaining rolls of steel which are not decarburized.
According to the invention, heating of the drawn roll is stopped at a temperature of between 620xc2x0 C. and 670xc2x0 C.
Likewise, according to this invention, the annealing temperature is 680xc2x0 C. or higher.
In accordance with the invention, during the annealing process the moisture content in the form of water vapour present in the atmosphere within the annealing furnace in direct contact with the rolls of drawn steel is continually analysed, as a result of which if the H2O level is sufficiently low heating is continued to the specified annealing temperature and the incoming flow of atmosphere into the annealing furnace is reduced, and if the level of H2O is higher than the value which is considered to be without risk of reaction with the carbon in the steel, the incoming flow of atmosphere into the annealing furnace is increased.
The process described above can likewise be applied to the annealing of carbon steel sheet coils when it is desired to avoid the loss of carbon from the sheet, and likewise this process must also be regarded as an object of this invention.
The process according to this invention will be described in greater detail below with the help of the accompanying drawings. It should however be understood that these drawings show a particularly preferred embodiment of the process according to the invention which should not be regarded as limiting it in any way.